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Issue A&A
Volume 471, Number 2, August IV 2007
Page(s) L47 - L50
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20078071



A&A 471, L47-L50 (2007)
DOI: 10.1051/0004-6361:20078071

Letter

The long-term stability of the visible F corona at heights of 3-6 R $\mathsf{_\odot}$

H. Morgan and S. R. Habbal

Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
    e-mail: hmorgan@ifa.hawaii.edu

(Recieved 13 June 2007 / Accepted 3 July 2007 )

Abstract
Context.CMEs can effect the distribution of dust grains in the corona. The brightness of the visible F corona is expected therefore to change as the frequency of CMEs varies with solar cycle.
Aims.We search for a variation in the F corona by comparing LASCO C2 observations from solar minimum and maximum.
Methods.An established inversion method is used to calculate the visible F corona brightness from LASCO C2 solar minimum observations made during 1996/10. Good agreement is found with the F corona brightness calculated from Skylab observations during 1973/05-1974/02 for heights of 3-6 $R_\odot$. The unpolarized brightness, which is dominated by the unpolarized F corona brightness at these heights, is obtained by subtracting many pairs of polarized brightness images from total brightness images and averaging over a solar rotation. We calculate the unpolarized brightness for both solar activity minimum and maximum.
Results.The unpolarized brightness, and therefore the F corona, remain virtually unchanged between solar minimum and maximum at heights above 2.6 $R_\odot$, despite the large change in the shape and activity of the corona. Using a simple density model, it is shown that the small variation in unpolarized brightness seen below 2.6 $R_\odot$ can arise from differences in the distribution of electron density, and therefore cannot be attributed to a variation in the F corona.
Conclusions.Despite the large rise in frequency of CMEs from solar minimum to maximum, the F coronal brightness, at heights of 3-6 $R_\odot$ in the visible, remains very stable.


Key words: Sun: corona -- interplanetary medium -- Sun: activity



© ESO 2007

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